Yes, Ebay can be a crap shoot. Card-stock models can be quite detailed, but they aren't as robust as plastic models. I have purchased several items from Ebay, but only stuff that I know the manufacturer of the item, and only from top rated sellers.

If the seller doesn't say, then it can be difficult. Name brand kits, like Pola and Faller are most certainly plastic kits. No name kits could be anything. Some guys are 3D printing plastic kits, while others are printing card-stock kits.